An existing apparatus for continuous vacuum-refining of metals comprises a cylindrical vacuum chamber accommodating a range of graphite trays arranged in succession one above another in a vertical plane, forming a column and adapted to receive preliminary melted impure metal, a central heater with a current lead for feeding electric power for heating the metal accommodated in the trays and for evaporating low-boiling impurities therefrom, and perforated shields for trapping impurity vapors referred to hereinafter as volatilized impurities liberated during evaporation, for condensing and draining the impurities into a condensate tank. Each shield is a cylinder mounted along the entire height of the tray column and not linked mechanically with the adjacent shield. Since the process of fabrication of the central coretype heater is a very complicated one, it has one and the same diameter all along its length, a feature precluding irregular distribution of generated energy along the chamber height.
A disadvantage of this apparatus resides in a low yield stipulated largely by the rate at which the volatilized impurities are drawn from the surface of molten metal accommodated in the trays. In the present-art apparatus the above process is determined by the rate of evaporation of impurities at their boiling temperature and by vapor diffusion in all directions. At present devices for intensifying the rate of withdrawal of volatilized impurities from the surface of molten metal are not available.
Another disadvantage inherent in the design of the vacuum-refining apparatus in current use lies in that it is impossible to ensure irregular distribution of energy, produced in the heater, along the height of the column formed by the trays mounted one above another.
The need for such an irregular distribution of energy along the height of the graphite tray column is dictated by the physics of the refining process.
In the first place, the refining process can proceed only at temperatures sufficient for boiling and evaporation of impurities present in metal being refined. Hence, the preliminary melted metal admitted into the vacuum chamber of the above apparatus should be heated to a temperature of evaporation of the impurities in the upper trays. Secondly, the upper trays accommodate the metal most contaminated with impurities.
Each unit of mass of impurities requires a certain amount of energy for its evaporation and since the mass of the impurities evaporating in the upper trays is a maximum one, naturally, it requires a larger amount of energy to be generated in the zone of location of the upper trays.
With the now-existing design of the central core-type heater the redistribution of the produced energy along the height of the tray column will require a heater of a variable (along its height) cross-section, and this presents a problem from the constructional standpoint and diminishes still more the inadequate durability of the heater.
As it has been stated above, the maximum amount of impurities evaporates from the upper trays. Over 60% of the total amount of such vapors are given off by the tray section which accounts for one-third of the height of the graphite trays in the upper part of the vacuum chamber. The middle third of the height of the graphite tray column accounts for almost 25% and the lower third for maximum 10% of the volatilized impurities liberated. This results in an increase in the partial pressure of the volatilized impurities in immediate proximity to the trays. The amount of energy given off by the volatilized impurities to the shields during their condensation is higher in the upper portion of the shields than in the middle and lower ones.
The inherent design of the present-art apparatus does not envisage the redistribution of energy generated by the heater along the column height; it ignores also the difference (along the column height) in the amount of energy given off by the volatilized impurities to the shields, which diminishes the stability of the refining process.
Finally, a fourth disadvantage of the apparatus design in current use lies in lower durability of the condensate tank which is due to the fact that the impurities withdrawn from the metal react actively with the material of the condensate tank at condensation temperatures.